As exemplified by the LED display described in U.S. Pat. No. 5,900,850, typical large tiled display devices require bulky structures to support the display tiles and keep them aligned.
U.S. Pat. No. 8,434,963, assigned to ORBUS, and EP 2 459 888 A1, in the name of the present applicant, give other examples of metallic support structures made of pultruded beams. Both documents describe solutions to problems such as axial and planar alignment of display tiles and illustrate problems such as assembly and disassembly of the support structure and concealment of the support structure.
US 2010/0135032 and US 2007/0218751, both assigned to Element Labs, Inc., are more particularly concerned by hanging tiled displays. They offer solutions to simplify assembly and maintenance of such displays but these solutions still present challenges at the time of assembly and disassembly.
EP 1 650 731 A1, in the name of the present applicant, discusses the mounting and fastening of one display element or tile to a display structure. EP 2 110 801 A2, in the name of Element Labs, Inc., is concerned with the fastening of the display element or tiles to a support structure and the alignment of the display tiles. While the solutions proposed in both documents simplify the assembly and servicing of the display element, both the fastening means and the support structure remain cumbersome.
One of the disadvantages of conventional LED display screens is that they are large, thick and heavy. For example, a 1 m2 8×8 LED display module 8 (2088AEG) weighs about 24.6 kg. Strong and heavy frames or supports are needed to support these LED display screens to ensure safety in the assembling process. The thickness of a conventional LED display screen is in the range of 5 cm to 50 cm. These conventional LED display screens are made of rigid PCB and can only be mounted on flat surfaces.
In one embodiment of EP 2 023 391 A2, a flexible LED screen may include a fixation layer coupled to a rear surface of the flexible printed circuit board to facilitate the fixing of the flexible LED screen to a support structure.
The fixation layer includes a plurality of openings sized and shaped to allow the integrated circuits to be situated therein. The fixation layer may be made of magnetic rubber and may have a thickness of about 1.5 mm. The fixation layer serves to fix the flexible printed circuit board on a wall or any supporting structure.
If fixed on a wall, connection means to power, data, and control signals must be provided.
If the flexible LED screens described in EP 2 023 391 A2 are tiles of a larger tiled LED display, the number of power and signal conducting cables and connectors increases rapidly. The number of cables may make the resulting structure very cumbersome. The cables may have to be hidden behind the display for aesthetic reasons, in which case a solution has to be found to guarantee that cables snaking between wall and the fixation layer will not affect the stability of the structure (e.g., by weakening adhesion to the wall) and will not introduce visual artefacts (by local bending of the tiles at those places where cables and tiles are in contact).
The cables may have to be routed between the tiles, in which case the dimensions of the cables will impose a minimum pixel pitch and resolution to avoid visual artefacts caused by pixel pitch variation between the outer pixels of adjacent tiles.
A screen of large dimensions is usually realized by combining several identical screen units of smaller dimensions. In most cases, a seam will be visible at the border between two adjacent units or tiles. Those seams have to be kept as discrete as possible and are often painted in black. To make things worse, thermal expansion can cause the seams to evolve differently across the screen with serious consequences for the visual perception of the displayed image. One of the main specifications of display quality is uniformity in color and brightness. For a tiled display, obtaining color and brightness uniformity is often even more difficult, because the tiles and their seams form a regular structure, which is very easily detected by the human eye. It is known that if a human eye observes a uniform plane, even the smallest local non-uniformities, such as a small variance on the mechanical seams, become visible.
Flexible LED strips are now available off-the-shelf (see FIG. 1a). They consist of a flexible substrate with adhesive tape on one side and LEDs and conducting tracks on the other side. The strips can be glued even to irregular surfaces. The flexible substrate is of the type commonly found in electronic appliances. The strips are available as 5 meter rolls. Realizing a display with N>1 rows of M>1 LEDs can be considered easy if there is a wall or surface available to glue the strips. Each strip will also have to be connected to a controller or the strips will have to be daisy-chained with ad-hoc cabling that is likely to be as cumbersome as that of the solutions previously described. The control of the pixel pitch between two parallel strips can be difficult to guarantee, which means that visual artefacts are likely to affect the display synthesized with the N>1 parallel strips. EP 1 716 553 A1 discloses a flexible tiled display 50 (see FIG. 1b) that solves some of the problems encountered with the LED strips. Flexible strips 30 comprising Light Emitting Diodes (LED) are cut so that each comprises a given number of LED modules 40 and/or has the required length for the flexible display 50. The strips are placed parallel to each other on a peripheral flexible circuit 60. The flexible circuit 60 consists of a flexible substrate (made of e.g. polyimide or PVC) on which electrically conducting tracks 22 have been formed. The tracks 22 connect the flexible strips 30 to supply and control circuits 20 via solder point 21 between tracks 22 on the substrate 60 and tracks 31 on the flexible strips 30. The tracks 22 are located on the periphery of the flexible substrate 60 and the flexible display 50. In essence, the display 50 consists of several tiles 30 spread over a single substrate 60.
The problems with the display proposed in EP 1 716 553 A1 are:                (a) The length L of the portions of conductor strips 31 on both extremities of flexible strips 30 must be at least as long as the width W of the bundle of tracks 22 to allow connections between all tracks 31 and tracks 22. The ribbons being produced continuously and cut to length to fit on the circuit 60, this means that the distance between pixel modules 40 on the flexible strips 30 will have to be equal or larger than W. The achievable resolution is therefore limited by the width of the tracks 22. Indeed, if two tiles 50 were assembled side by side to realize a larger tiled display, L and W have to be equal in order not to introduce visual artefacts caused by variation in the distance between pixels on the adjacent edges of the two tiles.        (b) The flexible strips 30 must be almost as long as the display 50 itself. For large displays 50 this may be a problem. Indeed, flexible LED strips are available as rolls of e.g. 5 meter or 10 meter long. A display with dimensions larger than 5 or 10 m would therefore require the tiling of at least two displays 50 as described in EP 1 716 553 A1, leading to the assembly problems discussed earlier. There would also be a problem between tiles. The distance between tiles would be at least twice the width W of the bundle of conductor strips at the periphery of the displays 50. The pixel pitch across the tiled display would therefore not be constant and lead to undesirable visual artifacts.        (c) If one LED module is defect, on-site servicing will require the replacement of the entire tile 30 on which the defect LED is located as is usually the case for tiled displays (see e.g. US RE 41,603 E). The dimension of the tile 30 being always as large as either the length or width of the display 50, this is not practical for displays several meters across, in particular if the replacement has to be done on site.        (d) The tracks 31 on tile 30 have to conduct power to all the LEDs on that tile. This will either require increasing the thickness of the tracks 31 or changing the material of which they are made (impacting flexibility and/or cost) or their width (which may require increasing the width of the flexible strip 30 and decrease the resolution).        (e) The connections (solder points) 21 between tracks 22 and 31 will have to conduct the current needed for all the LEDs on tile 30. For large displays this may lead to reliability issues as current increases linearly with the number of LED on the tile 30.        (f) Even if the strips 30 can be made long enough to extend from one end to another of a very large tiled display 50, the substrate 60 still has to be of one piece. Any problem with the substrate 60 would then require a disassembly of the entire display. For very large displays, this can be too unpractical and/or costly.        
US patent application publication no. 2009/0322651 A1, in the name of Thomas Tennagels, relates to a display device for the presentation of visual content at big events and to a display method for the presentation of such content. The device comprises a plurality of display elements (e.g. LEDs or clusters of LEDs) connected with carrier elements, the display elements being disposed in rows. Furthermore, the display device comprises a carrier rail for mechanically fastening the carrier elements and to supply the display elements with voltage and control signals. The rows of the display elements extend at least partially in several directions, starting from the carrier rail. The control signals originating from the carrier rail are transmitted via a feed element in a direction of a display element row, wherein the signals are deflected at a first end of the display element row through a first end cap in the direction of a second end of the display element row, then deflected at the second end of the display element row by a second end cap in the direction of the carrier rail, and deflected back into the carrier rail via the feed element. In the disclosed system, the carrier elements are rigid tubes and the carrier rail is a single piece, common to the entire display device.
In DE 3633565 A1, in the name of Licentia GmbH, in order to fit integrated circuits onto a substrate which is provided with conductor tracks, and to make contact with said integrated circuits, it is proposed additionally to fit a flexible supply lead and then to connect the connecting tabs of the integrated circuits electrically and mechanically to the conductor tracks of the substrate by means of pressure and a non-conductive adhesive, and to connect them to the conductor tracks of the supply cable by soldering. This method does not provide a solution to the problems associated with large modular display systems.
European patent application publication no. EP 1 469 450 A1 in the name of the present applicant is concerned with OLED display devices, which may be tiled such that a seamless large-screen display is achieved, and which may be individually back-coupled by means of connections to respective backing elements such as PCBs. This document is not concerned with flexible display tiles, and it does not disclose using a common substrate carrying multiple display tiles.